In certain circumstances it might be preferable to use a free running reference clock to clock the receiver PLL in the GTPE2 transceiver of the CPRI slave core. This removes the need for the external jitter-removal PLL to be free-running in the absence of a reference signal.
When the free_running_rx_reference generic is set to TRUE, both PLL0 and PLL1 in the GTPE2 transceiver are used. The receive datapath uses PLL1 . This PLL is clocked from a crystal oscillator. The transmit datapath uses PLL0 . This is clocked from the recovered clock through an external jitter removal PLL. This PLL need not be free running in the absence of the reference clock from the GTPE2. The pll_select input to the core should be tied to 11 in this mode.
This Figure shows the clocking for a system supporting 6,144.0 Mb/s.
